In electrophotographic and electrostatic printing processes (collectively electrographic processes), an electrostatic image is formed on the surface of a photoreceptive element or dielectric element, respectively, with a toner. In electrostatic printing, a latent image is typically formed by (1) placing a charge image onto a dielectric element (typically the receiving substrate) in selected areas of the element with an electrostatic writing stylus or its equivalent to form a charge image, (2) applying toner to the charge image, and (3) fixing the toned image. An example of this type of process is described in U.S. Pat. No. 5,262,259. In electrophotographic printing, also referred to as xerography, electrophotographic technology is used to produce images on a final image receptor, such as paper, film, or the like. Electrophotographic technology is incorporated into a wide range of equipment including photocopiers, laser printers, facsimile machines, and the like.
Electrophotography typically involves the use of a reusable, light sensitive, temporary image receptor, known as a photoreceptor, in the process of producing an electrophotographic image on a final, permanent image receptor. A representative electrophotographic process involves a series of steps to produce an image on a receptor, including charging, exposure, development, transfer, fusing, cleaning and erasure.
In the charging step, a photoreceptor is covered with charge of a desired polarity, either negative or positive, typically with a corona or charging roller. In the exposure step, an optical system, typically a laser scanner or diode array, forms a latent image by selectively discharging the charged surface of the photoreceptor in an imagewise manner corresponding to the desired image to be formed on the final image receptor. In the development step, toner particles of the appropriate polarity are generally brought into contact with the latent image on the photoreceptor, typically using a developer electrically-biased to a potential opposite in polarity to the toner polarity. The toner particles migrate to the photoreceptor and selectively adhere to the latent image via electrostatic forces, forming a toned image on the photoreceptor.
In the transfer step, the toned image is transferred from the photoreceptor to the desired final image receptor; an intermediate transfer element is sometimes used to effect transfer of the toned image from the photoreceptor with subsequent transfer of the toned image to a final image receptor. In the fusing step, the toned image on the final image receptor is heated to soften or melt the toner particles, thereby fusing the toned image to the final receptor. An alternative fusing method involves fixing the toner to the final receptor under high pressure with or without heat. In the cleaning step, residual toner remaining on the photoreceptor is removed.
Finally, in the erasing step, the photoreceptor charge is reduced to a substantially uniformly low value by exposure to light of a particular wavelength band, thereby removing remnants of the original latent image and preparing the photoreceptor for the next imaging cycle.
Two types of toner are in widespread, commercial use: liquid toner and dry toner. The term “dry” does not mean that the dry toner is totally free of any liquid constituents, but connotes that the toner particles do not contain any significant amount of solvent, e.g., typically less than 10 weight percent solvent (generally, dry toner is as dry as is reasonably practical in terms of solvent content), and are capable of carrying a triboelectric charge. This distinguishes dry toner particles from liquid toner particles.
A typical liquid toner composition generally includes toner particles suspended or dispersed in a liquid carrier. The liquid carrier is typically nonconductive dispersant, to avoid discharging the latent electrostatic image. Liquid toner particles are generally solvated to some degree in the liquid carrier (or carrier liquid), typically in more than 50 weight percent of a low polarity, low dielectric constant, substantially nonaqueous carrier solvent. Liquid toner particles are generally chemically charged using polar groups that dissociate in the carrier solvent, but do not carry a triboelectric charge while solvated and/or dispersed in the liquid carrier. Liquid toner particles are also typically smaller than dry toner particles. Because of their small particle size, ranging from about 5 microns to sub-micron, liquid toners are capable of producing very high-resolution toned images.
A typical toner particle for a liquid toner composition generally comprises a visual enhancement additive (for example, a colored pigment particle) and a polymeric binder. The polymeric binder fulfills functions both during and after the electrophotographic process. With respect to processability, the character of the binder impacts charging and charge stability, flow, and fusing characteristics of the toner particles. These characteristics are important to achieve good performance during development, transfer, and fusing. After an image is formed on the final receptor, the nature of the binder (e.g. glass transition temperature, melt viscosity, molecular weight) and the fusing conditions (e.g. temperature, pressure and fuser configuration) impact durability (e.g. blocking and erasure resistance), adhesion to the receptor, gloss, and the like.
U.S. Pat. No. 4,547,449 to Alexandrovich, et al. discloses liquid electrographic developers comprising an electrically insulating liquid carrier, toner, a charge-control agent and a charging agent. The charge-control agent is a carrier-soluble, addition copolymer of a quaternary ammonium salt monomer, a monomer having —COOH, —SO3H or —PO3HR acidic function wherein R is hydrogen or alkyl, and a solubilizing monomer. The charging agent is a carrier-soluble, addition polar copolymer. The disclosed developers are stated to exhibit improved replenishability as evidenced by reduced buildup of charge in the developers during the course of use and repeated replenishment. Specifically, this patent noted that the prior art exhibited drawbacks relating to the stability of their charge as they are used through a number of copy sequences. In particular, the charge of the developer per unit of mass of dispersed toner of the prior art increases, indicating that the quaternary ammonium charge-control copolymer deposits on an electrostatic image at a lower rate than the toner. This uneven depletion rate and consequential increase in charge per unit mass in the developer presents difficulty in developer replenishment and causes nonuniform image density from copy to copy. The invention as described therein is asserted to stabilize the charge of the developer per unit mass of toner is so that, after a period of use, the buildup of charge per unit of mass is significantly reduced. Such stability is stated to be achieved when the quaternary ammonium salt charge-control polymer in the developer composition contains an insolubilizing monomer having an acidic function selected from the group consisting of —COOH, —SO3H or —PO3HR acidic function wherein R is hydrogen or alkyl.
U.S. Pat. No. 6,018,636 to Caruthers discloses an imaging system wherein changes in toner developability of toners in a liquid toner system are determined and compensated for by sensing the toner concentration and liquid toner volume in a tank, based on changes in the toner concentration and toner mass in the tank. Based on measurements made of the toner and/or a test printed image, adjustments can be made, such as creating a new voltage differential or adding toner and/or liquid carrier material to the tank.
U.S. Pat. No. 5,722,017 to Caruthers discloses a liquid developing material replenishment system wherein an apparatus for developing an electrostatic latent image with a liquid developing material includes a liquid developing element for providing a supply of operative liquid developing material to the developing apparatus, and a liquid developing material supply is coupled to the liquid developing material element for providing a supply of liquid developing concentrate to the liquid developing material element for replenishing the supply of operative liquid developing material in the liquid developing element. A developed image having a large proportion of printed image area or having substantially a single color will cause a greater depletion of marking particles and/or charge director in the liquid developing material supply tank as compared to a developed image with a small amount of printed image area or of a single color. This patent explains that                while the rate of the replenishment of the liquid developing material may be controlled by simply monitoring the level of liquid developer in the supply element 116, in advanced systems the rate of replenishment of the liquid carrier, the marking particles, and/or the charge director components of the liquid developing material is controlled in a more sophisticated manner to maintain a predetermined concentration of the marking particles and the charge director in the operative solution stored in the supply element 116. One exemplary replenishment systems of this nature include systems which measure the conductivity of the operative liquid developing material and add selective amounts of charge director compound to the element as a function of the measured a conductivity, as disclosed in detail in U.S. Pat. No. 4,860,924, incorporated by reference herein. Another system of this nature is disclosed in commonly assigned U.S. patent application Ser. No. 08/551,381, also incorporated by reference herein, which describes control of the amount of carrier liquid, charge director and/or marking particles in a liquid developing material element in response to the amount of each component depleted therefrom as a function of the number of pixels making up each developed image.See column 14, line 48 to column 15, line 3.        
U.S. Pat. No. 4,860,924 to Simms, et. al. discloses a copier wherein charge director is supplied to a liquid developer in response to a conductivity measurement thereof. Toner concentrate deficient in charge director is supplied to the liquid developer in response to an optical transmissivity measurement thereof. Conductivity is measured by a pair of spaced electrodes immersed in the developer and through which a variable alternating current is passed. A variable capacitor neutralizes the inherent capacitance of the electrodes. A phase sensitive detector is provided with a reference voltage having the same phase shift as that caused by capacitive effects. The conductivity measurement is corrected in response to a developer temperature measurement.